LiPo internal resistance measurement tool

This is a scratch-build meter for measuring the internal resistance of Lithium Polymer cells. [Bleuer Csaba] uses the LiPo cells for RC vehicles and thet take quite a beating from the motors they’re supplying. This means that he only gets about 100-200 cycles out of each cell. To figure out where one is in its life cycle you can measure the internal resistance where a rising resistance indicates greater age. [Bleuer] mentions that you can buy a meter to do this for you, but what fun is that?

Since he’s rolling his own tool he defined his own parameters for the readings. After experimenting with different loads driven for different test periods he was able to extrapolate an equation that estimates the resistance measurement. As you can see in the clip after the break, this happens very fast. All he has to do is connect the cell and press one button. The measurements are made and various data points are displayed on the quartet of 7-segment displays.

[Bleuer Csaba]: “The battery is fully charged, cell voltage is about 4.2V. The unloaded cell voltage is measured. Than it is loaded with 1ohm for 2.5ms, and the loaded cell voltage is measured.”

The method is flawed.

As explained in the article above, the voltage measurement is made between two different load currents, small and large. The open voltage of the cell tells you nothing. Ultimately, you need two datapoints on the current-voltage curve of your power source. Once you have them, the resistance is simply the slope of the line between them.

Suppose you draw 1 Amp and the voltage at the terminals is 4.1 Volts. Then you draw 10 Amps and the voltage drops to 3.5 Volts. You divide the difference in voltage by the difference in current, and you have the resistance. In this case it would be 67 mOhm.

However, as the article states, resistance does not reveal the state-of-health of a battery. It is useful for spotting an already failed cell, or a counterfeit, or be used to match cells in a pack so they’d stay in balance under load and charging.

Right, but the open voltage depends on your state of charge and other effects, because the battery is not an ideal power source. When the battery is at rest, the electrochemical reactions inside charge up the internal capacitance to a higher voltage than what it can sustain, unless your load happens to be extremely tiny – much less than the internal leakage of the battery.

Be careful. Given enough idle time even a dead pack will recover the open voltage to its nominal value. A small load should be applied for the first data point. In my experience small cheapo DMMs may even load it, but the data loggers we use do not provide any load so a discharged battery will go down to 3.2 and then recover to 4.5 after being discharged.

In other words, the V-I relationship for any real battery is only linear up to a point. It is best estimated at the relevant loads you’re going to use, because the battery’s ability to source nanoamperes of current compared to 100 Amps is hardly relevant.

There are LiPo chargers on the market that measure Rin. No one knows what do they measure exactly, but they give a number in mohms. Even the good brands do not define this measurement on the net.

I have defined it: FULLY CHARGED battery is discharged through 1ohm. You are right, that the idle voltage has not too much meaning, but adding that the battery is fully charged defines the measurement better.

Additionally before the measurement begins it is loaded with the dividers. 20kohm/cell. That is not zero. Although it adds some error, because I do not calculate with this current.

This tool is only for LiPos. Forget the things you know about lead-acid.

According to the comment below: The word “order” is not in my dictionary. :-)

>” You are right, that the idle voltage has not too much meaning, but adding that the battery is fully charged defines the measurement better. ”

There is actually no real limit for when a lithium battery is full. It depends on how fast you want to destroy your battery, and cells can be charged right up until they burst into fire. The higher you charge it, the less charging cycles you will get out of the cell, and vice versa.

>”This tool is only for LiPos. Forget the things you know about lead-acid.”

As you may notice, the first graph in the article shows a lithium-ion battery, not a lead acid battery. When lithium batteries grow old, the lithium is permanently deposited onto the electrodes as metal and no longer takes part in the operation of the battery. This somewhat reduces the ability of the battery to supply current because there’s less lithium ions available, which is what I think you’re really measuring instead of the Rin. The full cell voltage is simply a constant that scales your measured values.

The ability to source current does not fully correlate with the capacity of the cell, and it is subject to change between different cells of different manufacture or batch, so you cannot reliably tell when a battery has gone through its cycles.

>”Additionally before the measurement begins it is loaded with the dividers. 20kohm/cell.”

20k per cell at 4.2 volts is 210 microamperes. It will take quite a while to discharge the internal capacitance of the cell even if it’s just 1 µF. In fact, your RC time constant would be 20 seconds.